Have you ever had anyone tell you that they feel the "good kind of tired" while you are feeling the much-more-common bad kind of tired? How do you experience having a good appetite versus just being hungry? To put in scientific terms, how do we judge valence?

What Valence Isn't

Valence isn't the term used for the intensity of emotion; that's arousal. You can have an equally positive response to a warm cookie and winning the lottery, but your degree of arousal will probably be higher for the lottery. Nor is valence dependent on what kind of emotion it is. When you hear a slightly suspicious noise in the kitchen at night you feel fear, and when you find out that your best friend can't be at your birthday, you feel disappointment, but the general negativity of the emotions are the same. Finding out how valence, arousal, and variation of emotion work together to make a functional human being is one of the goals of psychologists and neuroscientists alike.

Valence, Arousal, and the Brain

Valence is the term used for the spectrum of emotions from extremely positive to extremely negative. Although we spend much of our lives trying to read each other's faces, we're often pretty bad at it. We're also often bad at reading our own emotions. While some people can differentiate between sadness, guilt, anger, both in their own heads and in other people's expressions, other people only read all of those emotions as negative. They read joy, satisfaction, exhilaration — in themselves and others — all as positive. Valence, then, is the basic way to steer people one way or another, away from any negative and towards any positive.

When it comes to arousal — the intensity of an emotion — neuroscience indicates that valence might be less important than we think. The amygdala is the part of the brain that processes emotions. Over the years, scientists have found it easier to stimulate the negative side emotions with situations meant to repulse and disturb us. Positive emotions also get a rise out of the amygdala, but it's hard to determine whether positive or negative emotions do more to kick the amygdala into gear. To test how the brain responds to both differences of arousal and of valence, scientists had people look at positive or negative images while monitoring their brain activity.

Turns out, the amygdala gets engaged based on arousal. The feeling can be negative or it can be positive. Whether a person is shown a picture of snuggling kittens or a severed hand, the amygdala responds.

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But it isn't the only part of the brain that responds. The prefrontal cortex is a part of the brain we really don't want to lose, as it deals with planning and problem-solving, and any injury to the region is devastating. It turns out, this area of the brain might also tell you whether you feel good or bad. Different areas of the prefrontal cortex get going when different stimuli are presented. The areas are scattered, but there are distinct sections that distinguish between what you feel when you get a free plane ticket to Costa Rica and what you feel when you get there and a spider drops from a tree onto your face. Although it's your amygdala that tells you that both events were thrilling, it's your prefrontal cortex that tells you which one you wouldn't care to repeat.